The glass-ceramic technology has been the subject of many investigations over the last few decades. The work of many in this field has related to several different silicate systems types including the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 systems such as referred to in U.S. Pat. Nos. 2,971,853, 3,252,811, 3,282,712 and 3,380,818.
In these particular systems noted above, the compositions disclosed were those having particular photosensitive or transparent properties, or being particularly suited for lamp working or automatic equipment production.
Other investigations have occurred for various systems and the various modifications which effect strength, hardness, thermal expansion, and certain working properties.
More recently, the lower cost of the raw materials and technology and the apparent high strength of glass-ceramic materials have led to a quest for other applications of such materials, including the potential for application as a light silicate armour to resist high velocity projectiles.
Predicting ballistic resistance properties of glass-ceramic compositions has been found to be elusive since the desired projectile deformation is not necessarily consistent with standard measurements of hardness and strength between different known glass-ceramic compositions. Further, other necessary and desirable requirements for a material for such applications include a high degree of resistance to chemical attack such as by acid, bases or water. Further, the cost and ease of production of given shapes or configurations is an important consideration. The complex reactions occurring in these systems and the effect of various catalysts or additives which may surprisingly modify various properties disproportionately to the amounts added, make the quest for a suitable composition extremely difficult.
Prior to the present invention, a glass-ceramic system such as disclosed herein which possesses exceptional levels of these desirable properties has not been found.
While many prior publications and workers in the art have proposed compositions utilizing some combinations of the components used in the present invention, none have disclosed the same composition of components in the specific ranges disclosed herein nor obtained the characteristics of the material of the present invention.
For example, U.S. Pat. No. 3,282,712 discloses a Li.sub.2 O--Al.sub.2 O.sub.3 --SiO system in whch ZrO.sub.2 and TiO.sub.2 are used as additives or nucleating agents to promote crystallization. However, these disclosures teach a much higher Al.sub.2 O.sub.3 content and an end product having a very low co-efficient of thermal expansion. The final product in this patent includes a main crystalline phase having beta-eucryptite and beta-spodumene type crystals unlike the present invention. There is no disclosure teaching the ratio of the three main modifiers or nucleating agents used by Applicant.
U.S. Pat. No. 3,380,818 also discloses a similar composition range as the above patent. Similarly, it also teaches a higher range of Al.sub.2 O.sub.3 compared to the present invention and fails to disclose the ratio of modifiers or nucleating agents as taught by Applicant. Further, the end product is characterized by a low coefficient of thermal expansion primarily because it has a predominant amount of beta-eucryptite and beta-spodumene in the main crystalline phase which is undesirable in the present invention.